1. Field of the Invention
The present invention relates to an image reading apparatus for reading an image of an original document.
2. Description of the Related Art
Image reading apparatuses have become widely used as copiers, facsimiles, and image scanners. Image reading apparatuses perform shading correction in order to reduce density unevenness. In the shading correction, correction coefficients are generated by reading a white reference member (hereinafter referred to as a “white shading plate”), and image data is corrected using the correction coefficients. Since the white shading plate is a reference for white, the white shading plate should of course not become dirty, nor should a foreign substance such as dust be incorporated into or adhere to the white shading plate. Japanese Patent Laid-Open No. 03-9661 discloses that an original document placement face plate, which indicates the position where the original document is placed, is pasted to an original document platen with a double-sided tape, and the rear side of the original document placement face plate is defined as a white reference surface.
Recently, it is desired to further downsize image reading apparatuses. Downsizing of the image reading apparatuses may cause deterioration in accuracy of shading correction. The reason for the deterioration will be described with reference to FIGS. 9A and 9B. FIGS. 9A and 9B are diagrams illustrating a configuration of a white shading plate of a conventional apparatus. In FIG. 9A, the X-axis direction refers to a so-called sub scanning direction, and the Y-axis direction refers to a main scanning direction. The Z-axis direction refers to a direction perpendicular to an original platen glass 101. At the left end of the original platen glass 101, a white shading plate 302′, a sealing member 303′ such as a double-sided tape that is disposed around the white shading plate 302′, and a protection sheet 301′ for sealing the white shading plate 302′ are provided. The sealing member 303′ is a frame-shaped member whose inside is cut out.
The sealing member 303′ is an adhesive member that is slightly yellowish, thus having a lower light reflectance than the white shading plate 302′. Therefore, loss in the amount of light occurs in the sealing member 303′. As illustrated in FIG. 9B, the length L′ of a side of the white shading plate 302′ in the X-direction is shorter than the inside dimension L1′ of the cutout of the sealing member 303′ (the length in the X-direction), so that a “gap Δ” is generated therebetween. Also in this “gap Δ”, loss in the amount of light is caused.
Accordingly, due to the loss in the amount of light in the sealing member 303′ and the gap Δ, the reduction in luminance level (white level) near ends of the white shading plate 302′ (near the sealing member 303′) is greater than near the central part of the white shading plate 302′. That is, near ends of the white shading plate 302′ that are affected by the loss in the amount of light, it is impossible to determine the correction coefficients with high accuracy. Therefore, a region that is usable as a reference for white may be limited to the region near the central part of the white shading plate 302′. Although the problem can be solved by making the white shading plate 302′ wider in the X-direction, this makes it impossible to downsize the white shading plate 302′. Accordingly, in a conventional method, when accuracy of the shading correction takes priority, then downsizing is difficult, on the other hand, when downsizing takes priority, then the accuracy of the shading correction deteriorates.